public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null){
            return true;
        }
        return isSymmetricChild(root.left, root.right);
    }
    private boolean isSymmetricChild(TreeNode leftTree, TreeNode rightTree){
        if(leftTree == null && rightTree == null){
            return true;
        } else if(leftTree == null || rightTree == null){
            return false;
        }
        if(leftTree.val != rightTree.val) {
            return false;
        }

        if(isSymmetricChild(leftTree.left, rightTree.right) &&
                isSymmetricChild(leftTree.right,rightTree.left)){
            return true;
        }
        return false;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}